Luminous Dual Staff - 1H or 2h?

so the dual staff has the ball equipable projectile so it's a 1H.

That kind of logic / reasoning doesn't exist anymore.
That's like saying guns are 1h weapons, because Mechanics have an equipable projectile.

Does this make those 2h swords become 1h weapons after the recent patch in Kms where adventurers got some equipable projectile as well?

No, 2h weapons are still 2h weapons even with the 2ndary equip.